Ira:
You are correct, you are Wrong!
Air, Gases are COMPRESSIBLE! Liquids are NON-COMPRESSIBLE.
Liquids will transmit pressure equally in ALL directions and at the same pressure even through different/mixed size hoses. Physics - Fluid Dynamics 101.
Air on the other hand is compressible and also expandable and will expand and compress CHANGING its volume and pressure - Boyle's Law also a part of Physics 101.
So, the air bubble will cause the transducer and/or the gauge to pulse.
Barry
On Tue, Jun 20, 2017 at 9:10 AM, rampil <ira.rampil(at)gmail.com (ira.rampil(at)gmail.com)> wrote:

You are right about the static situation, it's a bit different dynamically.
Back in the day when I used to teach about the fluid dynamics
of invasive arterial blood pressure, I would note that
the distortion or "ringing" in the system was related to the
resonance of the system which in turn is a function of the
compliance of the tubing plus air bubbles, and the inertance
of the fluid column. When the driving frequency(pulsation of the
oil pump) gets close to the system's resonant frequency, the pressure
Oscillates wildly ( in a measurement system that has adequate frequency
response).

I will generalize (and guess) a bit to say that
1) the driving frequency of the oil pump, a multiple of RPM
is way higher than the responsiveness of mechanical
Oil pressure gauges to track the waveform even though you
would be able to see the ringing with an electronic transducer
and an o-scope.
2) air (increasing compliance) decreases the resonant
freq of the tubing system thus reducing the apparent
Oscillation on the gauge by pushing the resonant frequency
of the tubing well below the rpm harmonic.

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I am the OP....I suspect I have found the problem and will know tomorrow when I button up the plane and do a run up or perhaps a short flight. I will post my results. I doubt that it will involve anything very exotic, but we'll see.
JohnF

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Ira:
Sorry to say teach, wrong guess!
Well, not a guess, just over thinking a simple situation.
What about open and closed end resonance of the oil lines?
What about cavitation cause by bubbles in a bend, or the reduced flow especially at 90 Deg bends.
If you want to prove my reasoning true or disprove yours just add a length of oil line. That will change the 'frequency' as you stated and if you are correct. The pulsing will go away.
But, if you are wrong and if the air bubble remains then the problem will continue and your theory is proven wrong.
Think of it this way: Why does Rotax make a big deal about 'Burping' the system? Because air bubbles cause all these types of problems. If what you say is true, then there would be hundredths of plane owners complaining about the same problem. Both with and without Rotax engines.
Get rid of the air and the problem goes with it!
K.I.S.S. M.E.!
Barry
QA Engineer that punched holes in theories.

You are right about the static situation, it's a bit different dynamically.
Back in the day when I used to teach about the fluid dynamics
of invasive arterial blood pressure, I would note that
the distortion or "ringing" in the system was related to the
resonance of the system which in turn is a function of the
compliance of the tubing plus air bubbles, and the inertance
of the fluid column. When the driving frequency(pulsation of the
oil pump) gets close to the system's resonant frequency, the pressure
Oscillates wildly ( in a measurement system that has adequate frequency
response).

I will generalize (and guess) a bit to say that
1) the driving frequency of the oil pump, a multiple of RPM
is way higher than the responsiveness of mechanical
Oil pressure gauges to track the waveform even though you
would be able to see the ringing with an electronic transducer
and an o-scope.
2) air (increasing compliance) decreases the resonant
freq of the tubing system thus reducing the apparent
Oscillation on the gauge by pushing the resonant frequency
of the tubing well below the rpm harmonic.

Cavitation requires massive pressure changes usually caused by flow in
hydrodynamic systems, high speed flow and shear. There is essentially no
flow in a dead end tube to a pressure diaphragm. If there is a bubble,
maybe you will get a bit of to-and-fro, but not the kind of shear to create
cavitation. Same reason bends really don't affect pressure transducer lines.
Small pressure changes like 0 to 60 psi would have to occur with huge
acceleration to cavitate oil, a fluid designed to have high surface tension.
Adding a length of tubing increases inertance and a wee bit of resistance.
Burping these engines is all about refilling the reservoir and preventing
entrained bubbles from blocking oil channels in the block which would
depress lubrication and heat transfer. Nothing to do with resonance
phenomena.

Hi John,
You didn't say if you did a full oil purge? If you didn't you still need to do one before a flight. Yes air in the system can cause pressure swings and sometimes after a purge, but should go away.

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Thanks for your suggestions/ideas. As I suspected last evening at home while thinking over the problem it struck me that when doing the air purge after replacing fuel and oil hoses that upon reflection that evening that I had "pumped" a lot of oil out of the engine while doing the purging. I had originally put in 3-liters of new Aeroshell Sport Plus 4, but I suspected I had emptied too much while purging.

This morning I refilled the oil to the top of the flats on the oil dip stick, purged the oil system again, buttoned up the plane, and ran it up.

One thing I wanted to do is monitor the operation of the Thermostasis unit. It worked perfectly, and got me up to 50-degrees C in a few minutes, high enough to apply takeoff power if need be.

The oil pressure was rock solid, moving from 66psi/81-degrees F, to 55psi-122-degrees F in those 3 or 4 minutes.

Problem was low oil quantity...now fixed. Reminds me what a mechanic once told me: If you have a problem, look at the last thing you did.

Seems good advise still.

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Hi John,
Glad it worked out. I usually put in 3.5 liters for a good oil purge. One thing about the oil purge is you don't need a lot of oil to come out. A small quantity is fine. You're just trying to make sure the lifters have oil and there are no huge air voids in the engine. It doesn't take that much excess oil to do that. If you run low on oil during a purge you can pump air into the system and cause those air voids.

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Once again John...
K.I.S.S. M.E.
You can argue theory all you want but it ALWAYS come down to what works!
Barry
On Wed, Jun 21, 2017 at 1:20 PM, JohnF <n29cx(at)ridgeviewtel.us (n29cx(at)ridgeviewtel.us)> wrote:

Thanks for your suggestions/ideas. As I suspected last evening at home while thinking over the problem it struck me that when doing the air purge after replacing fuel and oil hoses that upon reflection that evening that I had "pumped" a lot of oil out of the engine while doing the purging. I had originally put in 3-liters of new Aeroshell Sport Plus 4, but I suspected I had emptied too much while purging.

This morning I refilled the oil to the top of the flats on the oil dip stick, purged the oil system again, buttoned up the plane, and ran it up.

One thing I wanted to do is monitor the operation of the Thermostasis unit. It worked perfectly, and got me up to 50-degrees C in a few minutes, high enough to apply takeoff power if need be.

The oil pressure was rock solid, moving from 66psi/81-degrees F, to 55psi-122-degrees F in those 3 or 4 minutes.

Problem was low oil quantity...now fixed. Reminds me what a mechanic once told me: If you have a problem, look at the last thing you did.

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